The present invention relates to a novel immobilized enzyme electrode which is effective in the measurement of the substrate concentration of enzyme and in the conversion of enzyme reaction energy into electrical energy. More particularly, the present invention relates to an immobilized enzyme electrode wherein an immobilized oxidase, such as glucose oxidase, amino-acid oxidase, xanthine oxidase or the like and a metal oxide, which is reduced through the enzyme reactions and is electrochemically oxidized (anodically oxidized) are combined with each other.
Conventionally, as an enzyme electrode using various immobilized enzymes of an oxidase, there is known an electrode which electrochemically measures the concentration of O.sub.2 to be consumed or the concentration of H.sub.2 O.sub.2 to be produced, in the oxidation reaction of a substrate AH.sub.2 to A by an oxidase as shown in FIG. 1, thereby to indirectly measure the substrate concentration.
For example, the specification of the U.S. Pat. No. 3,539,455 describes an enzyme electrode of a H.sub.2 O.sub.2 detecting system. In this example, H.sub.2 O.sub.2 is anodically oxidized directly on an electron-collector such as graphite or the like and the oxidation current of the H.sub.2 O.sub.2 is measured directly. Accordingly, the response time thereof is prolonged due to the diffusion delay of H.sub.2 O.sub.2. Also, the current tends to be affected by the dissolved oxygen concentration. At the same time, the current value to be obtained is small, with consequent limitation of the oxygen solubility.